The invention is based on a method for operating drawing presses which in each case produce a drawn part during each working cycle, in each case one blank being inserted during each working cycle into the drawing tool of the drawing press, which tool consists of die, punch and blank holder and being clamped in by the blank holder at the edge with a specific clamping force and the drawn part being subsequently drawn between die and punch. Such methods are generally known, for example, from the customary practice in pressing plants.
In hand-fed drawing presses the drawing process, which occurs in a timed sequence, is corrected on the basis of a continuous visual inspection of the drawn parts by the operating personnel and of an individual manual intervention in the adjustment of the blankholding force. This is therefore a case of an adjustment process in which the human being is included as an essential, process-determining element. Apart from the monotony associated with this system and the required constant attentiveness and responsibility of the operating personnel, drawn part errors resulting from an inaccurate or incorrect adjustment of the blank-holding force are often not promptly detected. Thus, despite a constant monitoring of the drawing processes, faulty drawn parts leave the drawing press and adversely affect the productivity of the drawing press. In automatically fed presses or in pressing trains, only random sample-like visual inspection is carried out so that, particularly in modern pressing plants, there is a greater risk of rejected parts than in plants which still have complete manual operation.
In an article by F. -J. Neff, "CNC and DNC operation in hydraulic presses" in the publication Werkstatt und Betrieb (Workshop and Plant), 119 (1986) 11, pages 947 to 949, the author reports on a system for automatic quality inspection in pressing plants with appropriately developed hardware and software for a largely optimized press operation. Displacement sensors and pressure sensors for slides and die cushions are integrated into the presses. As a result, the stroke/slide force curve for each individual workpiece can be measured and also displayed with a monitor. This actual-value curve can be compared for each individual workpiece with a workpiece specific reference course. At the start of production, the reference course is produced or empirically determined for a specific workpiece to be manufactured and the data are stored; in fact, for example the stroke/slide force curve of the first fault-free drawn part can be used as a reference course. By means of the prescribed procedure and other measures not reproduced here, rapid refitting of a press to other workpieces and a monitored, i.e. failure-free press operation, or press operation in which an alarm is automatically given in the event of a failure, is ensured. It is mentioned that rejected parts during press operation as a result of tool wear can arise as a result of quality changes on the workpiece with respect to dimensions or material or as a result of quality of the lubrication. By means of a repeated comparison, in a timed sequence, of the variation of the workpiece-individual stroke/slide force curve with the reference course, rejected parts can be detected automatically and early. In the event of a tolerance range which "accompanies" the reference course being exceeded or undershot, a fault is reported and the machine is deactivated so that, if appropriate, intervention by personnel can occur. The press itself which is monitored in such a way obviously operates, at least until the next failure, with a constant setting of all process parameters.
In another article by D. Bauer, G. Gucker and R. Thor, "computer-supported blank-holding pressure optimizes deep drawing" in the publication Bleche-BanderRohre (Sheet Metal, Strip Metal, Pipes) 5-1990, pages 50 to 54, the authors initially point out that for the deep drawing of fault-free parts it is necessary for the blank-holding force not to be allowed to undershoot a specific minimum value which changes as a function of stroke and not to exceed a specific maximum value which also changes as a function of stroke, the curves for the minimum values and maximum values behaving in a workpiece-dependent fashion. Excessively high blankholding forces lead to fractures on the drawn part, whereas a blank holder which is pressed on too weakly allows folds to arise. The article recommends deviating from the previously widespread variation of the blankholding force which had a more or less high degree of constancy and using a variation of the blank-holding force against the press stroke which is optimized in dependence on the type of workpiece, it being possible for such a non-constant blank-holding force variation to be made up from several sections of a constant and/or a linearly rising or descending course or from a functionally stipulated course. The desired-value variation for the blank-holding force can be optimized in various aspects according to the cited publication and, depending on the optimization objective, possibly also has a different appearance. For example, the blank-holding force variation can also be optimized with respect to the maximum drawn part quality, in which case it is also possible here again for different considerations, depending on the type of workpiece, to be emphasized, for example freedom from fractures or folds or avoidance of shrink marks. Instead, when optimizing the blank-holding force variation, the design of the drawing process can also be more significant, for example the increase in the acceptable drawing depth with the objective of possibly being able to omit a drawing stage or save on sheet metal or achieve a greater strength of the drawn part. Tribological considerations can also be included in the optimization of the variation of the blank-holding force. The optimized blank-holding force variation, once it has been ascertained for a specific workpiece, is then followed up in a closed-loop controlled fashion during each pressing cycle, the ascertained desired-value curve, with the exception of occasional, subsequent manual improvements, being, however, uniformly maintained.
Despite the use of a variation of the blank-holding force which is optimized to this extent and a corresponding closed-loop control in accordance with this variation, the aforesaid article does not go into details on an automatic detection of errors on the drawn part.
It is already known to monitor stamping processes or stamping tools acoustically (cf., for example, German Patent Document DE-A 3,938,854 or DE-Z Industrie-Anzeiger 17/1991, Pages 40 to 44). In this case, sensors applied to the tool are used to detect signal parameters, in particular the sound amplitude and its range of fluctuation, in temporal correlation with the stamping process, and to compare them with prescribed set values. In stamping processes, the breakage of a punch, for example, can be clearly discerned acoustically; edge chippings or cutting edges that have become blunt can also be detected by comparing the normal sound in the case of a perfect tool with an altered stamping sound. Acoustic monitoring of stamping processes is therefore essentially only a monitoring of the stamping tool, the process control of the stamping process being prescribed essentially completely and finally by the tool design and being virtually incapable of being influenced by machine setting which can be varied in a reciprocating fashion. To this extent, the stamping processes, some of which proceed with cyclic sequences of up to 700 strokes per minute, are not comparable with comparatively slow deep-drawing processes, in which the blank-holding force is slaved to a set characteristic which is to be varied or optimized in a time sequence, as the case may be, during each deep drawing process. Consequently, acoustic monitoring of stamping tools provides no stimulus with regard to automatic process optimization of deep-drawing processes.
An object of the invention is to improve the method of the generic type to the extent that, in the case of non-optimum setting of the process parameters or in the case of a failure which is caused for example by quality changes or lubrication changes on the part of the workpiece, the latter can be detected automatically and early, i.e. while the drawn part is still in the press, and a suitable correction of the set value of the clamping force of the blank holder can become effective immediately, i.e. for the next workpiece and can also be performed automatically.
This object is achieved according to the invention with the method of the generic type means of the following process steps:
before starting up production of drawing parts of a specific type on a specific drawing press and using a specific drawing tool the structure-borne sound of the normal variation in the drawing noise dependent on time or the pressing stroke, that is to say without the risk of the production of "fractures" and without the risk of the production of "folds", is determined by a sound emission analysis of the structure-borne sound caused by the drawn part during the drawing process in the drawing tool, and is stored as a reference sound in the form of data, furthermore, before starting up production of drawn parts of the specific type on the specific drawing press and using the specific drawing tool a reference course of the sound components is respectively determined from this normal drawing noise for the sound components of periodic and of stochastic sound components and stored as data,
during the production of drawn parts of this type on the specific drawing press and using the specific drawing tool, the drawn part quality is determined with regard to the criteria of "fractures", "acceptable" and "folds" and the ranges lying therebetween qualitatively are determined automatically and during each working cycle by means of a sound emission analysis of the structure-borne sound caused by the drawn part during the drawing process in the drawing tool,
the structure-borne sound signal being examined in each case in a wide frequency spectrum with regard to the simultaneous occurrence of amplitude discontinuities (r) which are abnormal with respect to the reference noise and it being concluded that there is a "fracture" in the case of the occurrence of such spectrally distributed amplitude discontinuities (r),
the structure-borne sound signal being examined, furthermore, in each case with regard to the temporal variation in the level of periodic sound components on the one hand, and of stochastic sound components on the other hand, and in the case of a characteristic deviation of the actual-value courses of the actual working cycle from the corresponding reference courses it is concluded that there are "folds", and
it being concluded that there is an "acceptable" drawn part in the absence in the sound emission analysis both of the signal characteristics indicating "fractures" and of the signal characteristics indicating "folds" referred to below for short as "damage signals",
in order to optimize the clamping force (F.sub.n) which can be set at the blank holder, the clamping force (F.sub.n) for the following working cycle is changed or maintained uniformly as a function of the detected drawn part quality of a drawn part drawn in a preceding working cycle, specifically
in the case of an incipient crack on a previously drawn drawn part--"fractured" drawn part quality--the clamping force (F.sub.n) for the new working cycle is lowered with respect to the value set in that case,
in the case of a fault-free drawn part--"acceptable" drawn part quality--the clamping force (F.sub.n) is maintained uniformly and
in the case of folding on a previously drawn drawn part--"folded" drawn part quality--the clamping force (F.sub.n) for the new working cycle is increased with respect to the value set in that case.
Before starting up production, the drawing noise for each type of a drawn part that is to be drawn which can be picked up by a vibration sensor that can be applied to the drawing tool, of unambiguously "acceptable" drawn parts is analyzed and a characteristic variation of the amplitude envelope and of the level characteristics of periodic sound components and of stochastic sound components is determined therefor. Limiting data for extreme values or tolerant ranges for curve shapes can be established from these data as set data, dependent on the drawn part, and stored as data. Comparing the corresponding actual data during production with the set data provides the possibility of an automatic fault detection on the drawn part with respect to the damage cases of "fracturing" and "folding" during the drawing process itself. Consequently, corrective interventions can be made promptly as appropriate so that the press can continue to operate in the event of failures and, at most, one faulty part or, in the case of serious failures, possibly two faulty parts are pressed and subsequently acceptable parts are produced again. By means of the automatic fault detection, the method of process optimization which was previously operated, that is to say controlled, manually and under human inspection, becomes a control process which proceeds automatically and in a closed cycle.
In an expedient embodiment of the invention, the time and/or the degree of the damage signal can also be detected within the respective working cycle, in which case the clamping force of the blank holder is changed to a greater extent when a damage signal occurs early or when a stronger damage signal occurs than when a damage signal occurs late or a weaker damage signal occurs.
Further expedient embodiments of the invention consist in the automatic detection of fluctuations of process parameters and/or of quality fluctuations of the semi-finished product, which fluctuations require in each case a corresponding adaption of the blank-holding force in order to achieve optimum process control. Fluctuations of this kind are caused in particular by changes in
the material strength of the sheet bars, PA1 the thickness of the sheet metal, PA1 the roughness of the surface of the sheet bars, PA1 the thickness of the lubrication film and PA1 the viscosity of the lubricant.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.